The present invention relates to a method and apparatus for integrating isochronous and asynchronous communication capability in a network and, more particularly, to a method and apparatus for utilizing a non-zero thresholding technique to enable isochronous and asynchronous information to be communicated over a network.
The challenge of providing integrated (i e., shared bandwidth) multiple media access lies in accommodating potentially disparate needs of various traffic types. For instance, voice communication often is characterized as isochronous because it utilizes a constant bit rate, presenting data to the channel at fairly regularly spaced intervals in evenly distributed amounts. On the other hand, data communication is generally characterized as asynchronous because it is bursty or widely varying in bit rate, occupying the channel at random intervals for un-prescribed lengths of time.
Integrating different modes of communication in a common framework offers numerous advantages. For example, integration:allows load balancing to be utilized to accommodate periodic shifts in service demand. Load balancing provides the ability to accommodate more voice traffic during commute/drive times and more data traffic during work and evening hours. Integration also enables phased introduction of service capabilities, i.e., it allows the service provider to leverage investment initially made for one purpose against new revenue opportunities as they emerge. Integration also facilitates hedging, or in other words, providing the ability to grow capacity in one market in the event that another market declines or fails to materialize.
Past attempts have been made to accommodate isochronous traffic on an asynchronous framework, such as voice over Internet Protocol (VoIP). However, such attempts have produced mixed results. The greater the channel capacity is relative to the traffic demand, the more capable an asynchronous framework is in accommodating isochronous traffic. As bandwidth demand increases, isochronous traffic suffers from the inadequacies of asynchronous transport, particularly its inconsistent delay characteristics. Therefore, the success of such attempts has been largely dependent on the channel load relative to capacity.
Likewise, a conventional multiple access arrangement, such as a code division multiple access (CDMA) arrangement, that is constructed on an isochronous framework compromises the quality of high-speed data services. The greater the channel capacity is relative to the traffic demand, the more capable the isochronous framework is in accommodating asynchronous traffic. However, as bandwidth demand increases, asynchronous traffic suffers from the inadequacies associated with using isochronous transport for asynchronous traffic.
The requirements for integrated multimedia extend beyond simply providing sufficient throughput to accommodate different traffic types in a manner best suited to the multiple access framework. The challenge also lies in adapting the framework to provide transport in a manner deemed viable and satisfactory across a range of service requirements.
In a conventional CDMA isochronous framework, users are separated on the channel by Walsh codes. The Walsh codes are a set of codes that are generated using a time-orthogonal Walsh function. During call set up, the user""s communication device is assigned a Walsh code. This Walsh code is then utilized by the receiver of the user""s communication device and by the transmitter of the base station to communicate. Therefore, the users communicating over a particular channel utilize the same frequency, but are separated on the channel by the Walsh codes. Each Walsh code being utilized on a particular channel corresponds to a separate sub-channel. The transmissions from the base stations to the mobile stations are modulated with the Walsh codes. At the receiver of the mobile station, bit decisions are made by a zero thresholding mechanism at the output of a correlator. The correlator correlates the received symbols and outputs a correlation value. The zero thresholding mechanism evaluates the correlation value to determine whether a binary 1 or a binary 0 was received.
Specifically, if the correlator output falls below a preselected zero threshold value, meaning that the correlator output is negative, a binary 0 is output to the receive buffer. If the correlator output is above the preselected zero threshold value, meaning that the correlator output is positive, a binary 1 is output to the receive buffer. Alternatively, a negative correlation value may correspond to a binary 1 and a positive correlation value may correspond to a binary 0, depending on how bits are mapped to the signal level on the channel.
This type of zero thresholding mechanism is satisfactory for isochronous communication, such as voice communication, because isochronous information is sent continuously at a constant bit rate. Therefore, a given sub-channel is always occupied during a given frame period. Consequently, the receive buffer should always be receiving either a binary 0 or a binary 1 during a given frame period. However, a zero thresholding mechanism is not well suited for asynchronous transport because data is not always being sent over a given sub-channel. As stated above, asynchronous traffic can vary in bit rate and can occupy the sub-channel at random intervals for un-prescribed lengths of time. Therefore, a given sub-channel is not always occupied. Utilizing a zero thresholding mechanism in this environment can result in the receive buffer being updated even when there is no data being sent, which can result in the receive buffer being updated with erroneous information.
One solution to the problems associated with using zero thresholding in an isochronous network being used to transmit asynchronous traffic is to xe2x80x9cstuffxe2x80x9d the channel with bits that indicate when no data is being sent. However, in a CDMA isochronous network, multiple users are using the same frequency to transmit data over several sub-channels that are separated on the channel by the Walsh codes. Each sub-channel contributes some degree of noise to the other sub-channels. Stuffing bits onto the sub-channels contributes noise to the network, thus adversely affecting the signal-to-noise ratio (SNR) of the network.
Accordingly, a need exists for an isochronous framework that is capable of accommodating isochronous and asynchronous traffic in a manner that meets the throughput requirements for all types of traffic, while efficiently sharing channel resources.
The present invention provides a method and an apparatus for providing a multiple access isochronous network that is capable of accommodating isochronous and asynchronous traffic. In accordance with the present invention, a non-zero thresholding mechanism is utilized at the receivers of mobile stations communicating over the network to determine whether a given sub-channel is idle, or whether the receive buffer of an active sub-channel is to be updated with a binary 0 or a binary 1. Utilizing the non-zero thresholding mechanism allows for the possibility that nothing is being sent over a particular sub-channel during one or more symbol periods. Utilizing the non-zero thresholding mechanism enables asynchronous and isochronous transport to occur on an isochronous framework without the need for stuffing bits onto the channel.
Each receiver of a mobile station communicating over the network comprises a correlator that correlates the incoming symbols and outputs correlation results. These correlation results are analyzed by the non-zero thresholding mechanism. In accordance with the present invention, the non-zero thresholding mechanism first determines whether sufficient energy has been detected on the sub-channel to indicate that a binary 1 or 0 possibly was sent. In order to accomplish this, a first, non-zero threshold value is compared to the correlation result. If the absolute value of the correlation output does not exceed the first non-zero threshold value, then a determination is made that the sub-channel is idle.
If the absolute value of the correlation output exceeds the first, non-zero threshold value, then a determination is made as to whether the correlation output corresponds to a binary 0 or a binary 1. A relatively large negative result at the correlator output, i.e., a result that has a greater negative value than a second, negative threshold value, is interpreted by the non-zero thresholding mechanism as a binary 0. A binary 0 is then passed along to the receive buffer as a valid data entry. Similarly, a relatively large positive result, i.e., a result that has a greater positive value than a third, positive threshold value, is interpreted by the non-zero thresholding mechanism as a binary 1. A binary 1 is then passed along to the receive buffer as a valid data entry.
The present invention utilizes the fact that a relatively low-energy non-zero result at the correlator output presumably corresponds to the Walsh code of that particular sub-channel being absent for a given symbol period. The correlation result is not equal to zero due to interference on the sub-channel contributed by other users of the network. In other words, relatively low energy on the sub-channel corresponds to no information having been sent, rendering that particular sub-channel idle for the associated symbol period. Correspondingly, no update occurs at the receive buffer until data transmission resumes. When data transmission resumes, it will be evidenced by substantial energy, either positive or negative, at the correlator output relative to the non-zero threshold.
By using the non-zero thresholding mechanism in this manner, an isochronous network may be used for communicating both isochronous and asynchronous types of traffic, without the need for stuffing bits onto the sub-channels and without degrading the signal-to-noise ratio of the network. These and other features and advantages of the present invention will become apparent from the following description, drawings and claims.